1. Field of the Invention
The present invention relates to wireless sensor networks and, more specifically, network wake-up range extension using edge devices that broadcast wake-up signals or provide auxiliary communication channels.
2. Description of the Related Art
A wireless sensor network (WSN) consists of a set of sensor nodes organized into a cooperative network. Each sensor node may contain processing units (e.g., a microprocessor, some DSP processors, or even a CPU), multiple types of memory to store programs, collected data, or even an operating system, multiple sensors to obtain data from the environment (e.g., a temperature sensor, a humidity sensor, etc.), a power source (batteries or an energy harvesting component) and a wireless transceiver. There are typically one or more data sinks in the network that must gather the data sensed by the nodes.
Prolonging the network lifetime is one of the key challenges in WSNs. Most sensor nodes are battery powered, and due to the size and/or cost constraints, the battery life of each sensor node is limited. As a WSN is a self-organizing ad-hoc network, the network lifetime is affected by the communication traffic of the network, the communication protocols, and the battery life of each sensor node.
One method to improve the network lifetime is to duty cycle the sensor nodes, such that the nodes are put into a sleep state periodically. However, this approach requires accurate synchronization among the sensor nodes, as one node can only transmit data to another while both sensor nodes are active. In addition, unnecessary idle listening by receivers that are not the target of a transmission wastes energy. Low power listening (LPL), also called preamble sampling, is another solution to improve network lifetime for WSNs. LPL aims to reduce idle listening in asynchronous protocols by shifting the burden of synchronization to the sender. However, idle listening is not eliminated. For both synchronous duty cycling and low power listening approaches, reducing the duty cycle of a node can increase its lifetime at the cost of increasing the delay in data delivery and reducing the packet delivery ratio.
The use of a wake-up radio (WuR) is another approach to increase network lifetime by employing additional wakeup radio hardware. A wake-up transmitter (WuTx) initializes the transmission by sending a wake-up signal. When a wakeup receiver (WuRx) receives this signal, it will trigger the sensor node to awaken it from its sleeping mode to start data communication. This on-demand wake-up can save the energy wasted by idle listening. Moreover, the wake-up radio approach has potential advantages over duty cycling in terms of delay, collision, overhead and protocol complexity.
There are two types of wake-up radios being developed: active wake-up radios and passive wake-up radios. An active wake-up radio sensor node requires a power supply for the wake-up circuit, but usually has a relatively long wake-up range. On the other hand, a passive wake-up radio sensor node only utilizes the energy harvested from the wake-up radio and does not dissipate any energy from the battery. However, as the energy harvested by the wake-up circuit is limited, passive wake-up radio sensor nodes operate over a shorter range of distances compared to active wake-up radio sensor nodes.
Radio-frequency identification (RFID) technology is one of the feasible approaches to achieve a passive wake-up radio. RFID systems use RF electromagnetic fields to communicate with tags for the purpose of identification. RFID systems are widely used for managing assets and people, as well as for tracking inventory by attaching tags to merchandise. RFID systems are generally composed of RFID tags, which store the ID information, and an RFID reader, which transmits the electromagnetic energy to power the tags as well as to access or modify the tag ID information. There are three types of RFID tags: passive tags, active tags and battery-assisted passive tags. Among these three types of RFID tags, passive RFID tags have the advantage of small size and low cost, and they have close to zero maintenance. Because of these advantages, passive RFID systems have been rapidly deployed in recent years. In particular, passive ultra-high frequency (UHF) RFID readers and tags communicate in the frequency band from 860 MHz to 960 MHz, where the tags communicate by backscattering the radio waves they receive from RFID readers
RFID has a well-defined communication protocol and uses energy harvesting to enable this communication. However, an RFID receiver (i.e., a tag) used in a wake-up radio sensor node can only achieve a relatively short wake-up range compared to the communication range of the sensor node. As a sensor node with a WuRx can start communication only after it is awoken, a longer wakeup range can improve the communication efficacy of a WSN equipped with WuRs. In addition, a longer range enables a wider range of WSN applications to benefit from the use of WuRs. Thus, there is a need for low energy consumption wake-up radio systems that can operate at longer ranges.